Abstract. The recently proposed "Respiration Index"
(RI = log PO2/PCO2) suggests that aerobic metabolism is
limited by the ratio of reactants (oxygen) to products (carbon
dioxide) according to the thermodynamics of cellular respiration. Here, we
demonstrate further that, because of the large standard free energy change
for organic carbon oxidation (ΔG° = −686 kcal mol−1), carbon dioxide can never reach concentrations that would limit
the thermodynamics of this reaction. A PCO2 to PO2 ratio of
10503 would be required to reach equilibrium (equilibrium constant,
Keq = 10503), where ΔG = 0. Thus, a Respiration Index
of −503 would be the real thermodynamic limit to aerobic life. Such a
Respiration Index is never reached, either in the cell or in the environment.
Moreover, cellular respiration and oxygen provision are kinetically controlled
such that, within limits, environmental oxygen and CO2 concentrations
have little to do with intracellular concentrations. The RI is fundamentally
different from the aragonite saturation state, a thermodynamic index used to
quantify the potential effect of CO2 on calcification rates, because of
its failure to incorporate the equilibrium constant of the reaction. Not only
is the RI invalid, but its use leads to incorrect and misleading predictions of
the threat of changing oxygen and carbon dioxide to marine life. We provide a
physiological framework that identifies oxygen thresholds and allows for
synergistic effects of ocean acidification and global warming.